1. Field of the Invention
The present invention relates to an apparatus and method for controlling a propagation direction of a small sector antenna used for a small wireless system having limited power and calculation capacity such as a mobile communications terminal or a sensor node of a sensor network.
2. Description of the Related Art
Unlike an existing network, a wireless sensor network is basically configured to aim at the automatic and remote collection of information, rather than being a means of communication, a configuration which is extensively utilized in various fields of application such as for scientific, medical, military, commercial purposes. The sensor network includes a plurality of sensor nodes that detect information through a sensor and transmit the same, and a sink node that transmits the information which has been collected through the plurality of sensor nodes to the exterior.
Each sensor node has a simple structure including a sensor detecting information, a processor processing the detected information, and a wireless transmission/reception unit transmitting the processed information. For the convenience of installation and use, each sensor is required to be designed to consume little power and be compact so as to operate for a long time with limited battery power. In order to satisfy the demand for low power consumption and compactness, each sensor node generally has a simplified function and structure, having limited calculation capabilities.
Meanwhile, in order to configure an antenna detecting a propagation direction or indicating a directional radiation direction, a phased array antenna or a wireless communication system supporting multiple ports has generally been employed. However, the phased array antenna and the wireless communication system consume much power, need to have a high calculation capability, and need to include a plurality of RF ports.
In comparison, the small wireless device having a low-power consumption and limited calculation capabilities like the sensor node mostly supports an RF output of a single port and needs to control only the direction of a limited number of sectors.
Thus, in the case of the small wireless device such as the sensor node, it does not use a plurality of radiators like the phased array antenna to control the propagation direction but to configure an antenna with a single radiator and a plurality of parasitic elements to adjust an electrical length of a parasitic element and uses the interference between elements.
FIG. 1 illustrates a small sector antenna generally used in a small wireless device.
With reference to FIG. 1, the small sector antenna 10 includes a single monopole antenna 11 and a plurality of parasitic elements 12 installed at regular intervals in a circular form around the monopole antenna 11. A reference numeral 13 in FIG. 1 denotes a disk-type metal ground on which the monopole antenna 11 and the plurality of parasitic elements 12 are fixed. In this case, the length of the monopole antenna 11, the length of the parasitic elements 12, the distance between the monopole antenna 11 and the parasitic element 12, and the thickness of the disk-type metal ground are designed to be λ/4 (λ: radio signal wavelength).
The radiation direction of the small sector antenna is controlled by changing an electrical equivalent length according to a change in capacitance of the plurality of parasitic elements 12.
In the related art, a varactor diode is commonly used as a means of controlling the capacitance of the plurality of parasitic elements 12, which, however, disadvantageously accompanies a complicated calculation to appropriately adjust a nonlinear corresponding relationship between a bias voltage and the radiation direction.
In addition, in order to convert a digital bit stream generated according to the complicated calculation into a bias voltage of the varactor diode, a digital-to-analog converter (DAC) must be provided to increase the power consumption.